Heterogeneous methanol synthesis is in today's practice carried out by reacting carbon oxides with hydrogen in the presence of copper-based catalysts according to the following equations:3H2+CO2=CH3OH+H2O   (1)2H2+CO=CH3OH   (2)
The methanol synthesis catalyst also catalyses the Water Gas Shift (WGS) reaction,CO+H2O=CO2+H2   (3)and the Reverse Water Gas Shift (RWGS) reaction,CO2+H2=CO+H2O   (4)
In large-scale methanol plants, the gas phase synthesis reactor is typically a cooled tubular reactor or a multistage adiabatic reactor. The typical temperature range for methanol synthesis is 200-300° C. Cooled reactors normally operate at approximately 250° C., whereas adiabatic reactors typically operate between 220° C. and 300° C. The reaction to methanol is strongly exothermic, and efficient heat removal is a problem. This limits the range of composition of the feed gas that can be treated in a tubular reactor, for example CO rich gases are very exothermic and difficult to process. Inefficient heat removal leads to hot zones in the reactor, and the catalyst may thus deactivate faster. A serious problem is by-product formation being promoted at high temperatures appearing particularly in the hot zones. For example the production of ethanol, and methyl formate increases at increasing temperatures. Alcohols, esters and ketones are difficult to separate from methanol-water mixtures being withdrawn from the process due to the formation of azeotropes.
With respect to pressure, the typical operating pressure in methanol synthesis is within 50-100 bar. Cooled reactors operate at a pressure of around 50 bar, whereas adiabatic reactors generally operate at a higher pressure, typically around 80 bar.
The synthesis gas used in methanol synthesis can be derived from natural gas either by steam reforming of natural gas or by autothermal reforming.
The conversion rate to methanol is determined by the thermodynamic equilibrium constant, which typically results in process effluent concentrations being in the range between 4% and 10% in all cases with only a partial conversion of the carbon oxides. Thus, to obtain maximum use of the produced synthesis gas, a recycle of the unconverted gas is necessary. A high recycle ratio ensures high conversion. A high recycle flow through the reactor increases the compression cost. Before recycling, the effluent gas is cooled and separated from the liquid product. The effluent gas can be enriched in hydrogen by employing, for example, membrane separation. The enriched gas is recycled to the methanol reactor. The inert level, in particular methane is kept under control by purge.
Beside of the above mentioned tubular methanol reactor being conventionally employed in the industrial production of methanol use of slurry bed methanol reactors have also been suggested in the literature.
The original invention of the slurry reactor was made by Chem Systems and is described in U.S. Pat. No. 3,888,896. Herein a preparation process is described using an inert organic liquid such as pseudocumene as the slurry medium.
A series of patents from Air Products (U.S. Pat. Nos. 4,628,066, 4,766,154, 4,910,277, 5,179,129 and 5,284,878) describe the application of a slurry reactor for methanol synthesis. These applications are characterised by the suspension of the methanol catalyst in an inert liquid, typically a high-molecular-weight hydrocarbon. Only partial conversion of methanol is considered here, as the reaction conditions are given as 30-100 bar and 210-250° C.
The finely divided catalyst must be separated from the liquid product. U.S. Pat. No. 5,520,890 by Den Norske Stats Oljeselskap A. S. describes an apparatus for solid-liquid slurry treatment. The process described in this patent provides a solution for the separation of fine catalyst particles from the liquid by operation in tubes including filtrate zones.
JP Patent No. 57126434 discloses a methanol preparation method from CO and/or CO2 in the presence of a water-soluble basic substance where water is used as slurry medium.
A completely different approach to methanol synthesis from alkyl formates was first described by J. A. Christiansen in U.S. Pat. No. 1,302,011. This approach is also described in U.S. Pat. No. 5,384,335 and consists of two distinct steps:
First Step:
Carbonylation of methanol to methyl formate, HCOOCH3(=C2H4O2), in a basic medium e.g.CH3OH+CO=C2H4O2   (5)Second Step:
Reduction of methyl formate to methanol on a Cu-based catalystC2H4O2+2H2=2CH3OH   (6)
Various solvents are described including the use of methanol as a solvent. The conditions under which the above reactions take place differ significantly from the conditions applied in industrial methanol synthesis. Reaction 5 requires a temperature much lower than 200° C., whereas reaction 6 is preferably carried out at much higher temperatures. In some processes the reactions are actually carried out in separate reaction vessels.
The most important difference between the above process and the industrial methanol synthesis is the role of CO2. In the above process CO2 acts as a poison for reaction 5, whereas in the heterogeneous synthesis of methanol (reaction 1) CO2 is a necessary reactant. H2O also acts as a poison in the above process.
A conversion not limited by the thermodynamic equilibrium can be obtained in the case of a methanol product removal under synthesis. U.S. Pat. No. 5,262,443 to Haldor Topsoe A/S describes such a process where methanol synthesis is carried out under condensing conditions in a fixed bed in a cooled tubular reactor. Methanol condensing conditions include a temperature below the critical temperature of methanol, i.e. 240° C. In addition, the methanol partial pressure calculated from the gas phase equilibrium constant is larger than the boiling pressure of methanol at the actual temperature. The process is most conveniently carried out with a stoichiometric synthesis gas being enriched in CO.
The removal of heat in the tubular reactor disclosed in the later patent and in tubular methanol reactors in general is one of the key problems of such a process. The most serious problem when employing fixed catalyst bed methanol reactors is formation of by-products being formed in hot zones of the bed caused by the highly exothermic nature of the methanol reactions in the catalyst bed.
In contrast to the above discussed prior art, the present invention is a preparation method for methanol at condensing reaction conditions wherein the methanol catalyst is suspended in methanol and water, and wherein the product being formed on the suspended catalyst (primarily methanol and water) continuously condenses and being absorbed in the suspension and thus makes up the suspension medium for the methanol catalyst.
When carrying out the methanol reactions in a catalyst suspension heat being produced during the reactions is effectively controlled. Formation of hot spots or hot zones a known problem in the fixed bed reactors is prevented since heat is transferred and distributed uniformly within the liquid. The efficient temperature control is necessary to reduce by-product formation as mentioned above and to minimise deactivation of the catalyst.